LAUSR

Abstract Dual-polarity characteristic magnetizations in red sandstones of the Early and Middle Eocene El Bosque Formation in central Chiapas, Mexico (D = 9.3° I = 23.6°; N = 19 selected sites, k = 32.6, α95 = 6.0°) indicate about 20° of clockwise rotation when compared with a coeval paleomagnetic reference pole in the North America craton. We interpret the rotation to result from traction between the base of the Chiapas Massif lithosphere and the low-angle subducting Cocos Plate. The northwest end of the massif overlies a shallower portion of the Cocos plate interface than the southeast end, which implies a greater amount and longer period of tractional coupling between the two plates in the northwest. We judge that the downdip, steep limb of the Cocos slab (beneath Gulf of Mexico) lies roughly in the regional mantle or hotspot reference frame. Assessments of Neogene North America-hotspot relative motion indicate an azimuth of ENE-WSW at 20 mm/yr, and thus we envisage that (1) the flat slab zone has been caused by the WSW telescoping of North America across the former geometry of the Cocos Benioff Zone, and (2) that the WNW end of the massif has undergone most tractional coupling with Cocos Plate than the ESE end. In turn, because the massif appears to have behaved as a rigid, unbroken body, this greater coupling has induced clockwise rotation that has been transferred as near-field shortening into the adjacent Sierra de Chiapas foldbelt.